Soil Erosion Risk Assessment in Uganda

Abstract

Land use without adequate soil erosion control measures is continuously increasing the risk of soil erosion by water mainly in developing tropical countries. These countries are prone to environmental disturbance due to high population growth and high rainfall intensity. The aim of this study is to assess the state of soil erosion by water in Uganda at national and district levels, for various land cover and land use (LCLU) types, in protected areas as well to predict the impact of support practices on soil loss reduction. Predictions obtained using the Revised Universal Soil Loss Equation (RUSLE) model indicated that the mean rate of soil loss risk in Uganda's erosion-prone lands was 3.2 t·ha −1 ·y −1 , resulting in a total annual soil loss of about 62 million tons in 2014. About 39% of the country's erosion-prone lands were comprised of unsustainable mean soil loss rates >1 t·ha −1 ·y −1 . Out of 112 districts in Uganda, 66 districts were found to have unsustainable estimated soil loss rates >1 t·ha −1 ·y −1 . Six districts in Uganda were found to have mean annual soil loss rates of >10 t·ha −1 ·y −1 : Bududa (46.3 t·ha −1 ·y −1 ), Kasese (37.5 t·ha −1 ·y −1 ), Bundibugyo (28.9 t·ha −1 ·y −1 ), Bulambuli (20.9 t·ha −1 ·y −1 ), Sironko (14.6 t·ha −1 ·y −1 ) and Kotido (12.5 t·ha −1 ·y −1 ). Among the LCLU types, the highest soil loss rates of 11 t·ha −1 ·y −1 and 10.6 t·ha −1 ·y −1 were found in moderate natural forest and dense natural forest, respectively, mainly due to their locations in highland areas characterized by steep slopes ranging between 16% to 21% and their high rainfall intensity, ranging from 1255 mm·y −1 to 1292 mm·y −1 . Only five protected areas in Uganda were found to have high mean estimated mean soil loss rates >10 t·ha −1 ·y −1 : Rwenzori Mountains (142.94 t·ha −1 ·y −1 ), Mount Elgon (33.81 t·ha −1 ·y −1 ), Bokora corridor (12.13 t·ha −1 ·y −1 ), Matheniko (10.39 t·ha −1 ·y −1 ), and Nangolibwel (10.33 t·ha −1 ·y −1 ). To manage soil erosion in Uganda's protected areas, there is an urgent need to control wildfires and human-induced disturbances such as timber harvesting and soil compaction from domestic animals. Our study analysis revealed that well-established terraces and strip-cropping could significantly reduce soil loss rates in Uganda's croplands by 80% (from 1.5 t·ha −1 ·y −1 to 0.3 t·ha −1 ·y −1 ) and by 47% (from 1.5 t·ha −1 ·y −1 to 0.8 t·ha −1 ·y −1 ), respectively, well below the sustainable soil erosion tolerance rate (1 t·ha −1 ·y −1 ) for land and water conservation.

Land Use Change Impacts on Water Erosion in Rwanda

“…Previous studies indicated that the rates of soil erosion are generally different for various types of LULC [54][55][56]. Related to this, the surface areas that are not prone to soil erosion, such as urban areas, bare rocks, glaciers, wetlands, lakes, rivers, inlands waters and marine waters [54,57], were not considered in this study.…”

“…Previous studies indicated that the rates of soil erosion are generally different for various types of LULC [54][55][56]. Related to this, the surface areas that are not prone to soil erosion, such as urban areas, bare rocks, glaciers, wetlands, lakes, rivers, inlands waters and marine waters [54,57], were not considered in this study. Therefore, the LULC maps of Rwanda for 2000 and 2015 ( Figure 2) provided by the Regional Centre for Mapping of Resources for Development (RCMRD) [58] at 30 m resolution were used to separate erosion-prone (forestland, grassland and cropland) and non-erosion prone areas (wetland, water bodies and built-up) in ArcGIS.…”

“…In both years, the highest soil loss estimates from evaluated LULC types (Table 4) are in the order of forestland > grassland > cropland and are in agreement with the work of Nyesheja et al [28] in the Congo Nile Ridge region of Rwanda, whose findings show a similar trend of forestland (104.93 t ha −1 y −1 ) > grassland (64.55 t ha −1 y −1 ) > cropland (19.1 t ha −1 y −1 ). Previous studies highlighted that soil erosion could surge from less than 1 t ha −1 y −1 in undisturbed forests to over 100 t ha −1 y −1 in disturbed forests [54,61,86]. In this study, the main reason for excessive soil loss observed in the forestland was primarily attributed by its exposure on steep slopes, consistent with results from the Lebir catchment in Malaysia, where the highest erosion rate (87.63 t ha −1 y −1 ) occurred in the high conservation value forests due to steep topography [87].…”

“…To minimize soil loss to a tolerable erosion threshold while alleviating its associated effects, soil conservation practices [11] have the potential to restore degraded soils, as well as improving water quality [3]. Practices such as terracing, contouring, and strip-cropping are the most commonly known conservation techniques established to mitigate water erosion [3,54]. Figure 9a shows that an area of 1018.7 km 2 of total cropland (13,582.7 km 2 ) in 2015 had a sustainable soil erosion rate (≤1 t ha −1 y −1 ), while 6071.5 km 2 (44.7%) suffered from damaging soil erosion rates (>10 t ha −1 y −1 ).…”

“…where SAN denotes the percentage of the sand content (0.05-2.00 mm diameter); SIL stands for the percentage of the silt content (0.002-0.05 mm diameter); CLA represents the percentage of the clay content (<0.002 mm diameter); C stands for the percentage of the organic carbon content; and SN1 = 1 − (SAN/100). The constant value of 0.1317 is used for conversion of K factor from the American unit system to International/metric unit system [34,54].…”